The present invention relates generally to medical systems and ophthalmic instruments. More particularly, the present invention relates to safety devices for use with laser eye surgery systems.
Photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) employ laser beam delivery systems for directing laser energy to a patient""s eye in order selectively to ablate corneal tissue to reform or sculpt the shape of the cornea thereby to improve vision. Present commercial systems employ excimer lasers, where the beams from the lasers are spatially and/or temporally integrated in order to form a beam typically having uniform characteristics. In particular, the beams are often integrated in order to display a flat intensity profile over a circular target region, often referred to as a xe2x80x9ctop hatxe2x80x9d profile.
Such uniformly integrated beams may be used in different ways in order to effect corneal ablation. In a first type of system, positioning of the beam is generally fixed and the beam has a cross-sectional area generally corresponding to an entire surface area of a surgical site on the cornea. Cross-sectional portions of the beam are then sequentially masked or adjusted so as to selectively vary the amount of energy exposure of different portions of the surgical site so as to effect the desired sculpting. This can typically be achieved by using an iris or other exposure control mechanism. While highly effective and relatively easy to control, employing a laser beam having a cross-sectional area generally equal to the area of the treatment or surgical site (typically having a diameter of 5.0 mm to 10.0 mm) often involves the use of relatively large amounts of energy. This is typically relatively expensive, and leads to relatively large laser systems.
As an alternative to such large beam diameter systems, laser xe2x80x9cscanningxe2x80x9d systems can be employed for corneal ablation. Such scanning systems typically employ a laser beam having a smaller cross-sectional area, thereby decreasing energy requirements. Accordingly, laser scanning systems delivering laser beams of relatively small cross-sectional area can be more economic to use and normally are of smaller construction than laser systems having larger diameter beams. However, the use of such small beams complicates certain aspects of the treatment protocols required to perform the sculpting. For example, to achieve a desired level of volumetric tissue removal or ablation from the eye, the treatment beam is scanned over or otherwise moved across the eye from one position to a next during the surgical procedure. Movement of the beam is typically achieved through motorized scanning mechanisms, devices, or the like. These scanning mechanisms often regulate the position of an optical element, e.g., the angle of a mirrored surface, or the lateral position of an offset imaging lens, or the like, so as to adjust the lateral position of the beam across the treatment site. In a related type of system, the laser beam is scanned over the corneal surface while varying the cross-section of the laser beam. Regardless, to achieve properly controlled laser exposure over the entire treatment site on the eye, the positioning of a scanning laser beam should be controlled accurately. If the beam resides at one position for too long, due to a jam or malfunction of the scanning mechanism, for example, the desired tissue ablation pattern may not be achieved. A jam of the scanning system may jeopardize the success of the surgery and could cause damage to the patient""s eye. Since the laser beam itself is not easily visible, malfunction of the scanning mechanism is not readily detectable by an observer.
Accordingly, it would be desirable to provide a device or subsystem for a laser surgery system which verifies the correct positioning or adjustment of the laser beam. Preferably, such a device should be able to be incorporated into a laser surgery system without interfering with the performance of the surgery. It would also be preferred that the device or subsystem be a cost-effective addition to a laser surgery system. It is envisaged that such a device or subsystem can find particular use in scanning laser beam systems. However, it will be appreciated that such a device or subsystem can also be used with large diameter laser beam systems.
The present invention provides systems, devices, and methods for verifying the scanning motion or adjustment of a laser beam. The present invention can advantageously be used in laser eye surgery where accurate control of the laser beam is crucial so as to ensure patient safety and successful vision correction. According to one aspect of the present invention a scanning laser beam system is provided which verifies that actual scanning of the laser beam across the treatment site on an eye follows a predetermined scanning sequence. Laser beam position feedback information is provided and compared with expected results. Should the feedback information be inconsistent with the expected results, the procedure is typically interrupted to inhibit injury to the patient resulting from, e.g., equipment failure.
Thus, according to one aspect of the present invention, a laser system is provided for sculpting a portion of the eye. The system includes a laser for generating a laser beam arranged to ablate eye tissue. The laser beam is typically used to sculpt the cornea of the eye (but may be applicable to other areas such as the iris, retina, or the like). A laser beam scanning mechanism is provided to scan the laser beam across the treatment area in accordance with a predetermined ablation pattern or scanning sequence. A motion detector or sensor operatively associated with the laser beam is provided at a position downstream of the scanning mechanism to verify the repositioning of the laser beam. Typically, the scanning mechanism verifies the repositioning of the beam by comparing expected energy readings with actual energy readings as the laser beam is moved successively from one lateral position to a next lateral position across the treatment site. Preferably, a beam splitter is used in the system to split the laser beam into a primary beam and a secondary beam. The beam splitter typically directs the primary beam towards the eye so as to ablate eye tissue and directs the secondary beam to the motion detector.
In one embodiment of the present invention, the laser system uses a motion detector having a photosensitive surface and a mask at least partially covering the photosensitive surface. The motion detector is thereby adapted to block or vary the exposure of the photosensitive surface to laser energy so as to register different energy readings as the beam is moved laterally across the surface in response to movement of the primary beam laterally across the treatment site. The mask typically comprises a covering blocking discrete portions of the surface to vary exposure of the surface to the laser beam as the laser beam is moved laterally thereacross from one position to a next position. The mask may have a configuration that blocks varying amounts of energy reaching the sensor as the beam is moved along an arbitrary X-axis of the sensor or an arbitrary Y-axis of the sensor. The position is verified by sequentially comparing expected energy values with the actual energy readings measured by the sensor as the secondary beam moves laterally across the photosensitive surface per the predetermined ablation pattern. Typically, the mask has a configuration that is asymmetric about the X-axis of the sensor, or the Y-axis of the sensor, or both. Preferably, the mask covers about 50% of the photosensitive surface.
According to another aspect of the present invention, a method is provided for verifying the motion of a laser beam across a treatment site on a patient""s eye. The method includes directing a laser beam onto a motion detector or sensor. The detector includes a photosensitive surface and a mask which causes the percentage of the photosensitive surface exposed to the beam to vary as the beam is scanned from one position to a next position during a corrective eye surgery scanning procedure. The expected energy values derived from the sensor are sequentially compared with actual energy readings measured by the sensor. If the readings do not correspond with the expected values, the beam may be interrupted or otherwise repositioned to inhibit the laser beam from harming the patient""s eye.